The present invention is directed to a carriage guide, particularly for typing element carriers for office machines, of the type composed of two guide rails extending parallel to a printing abutment and pairs of guide rollers mounted on an intersecting axis and connected with the typing element carrier, the typing element carrier centering itself by means of the guide rollers at one or both of the guide rails.
Carriage guides essentially constituted by guide rollers connected with a typing element carrier and guide rails provided in a machine frame are generally known in the art. In the development of such carriage guides it has been found that problems requiring special measures for their solution occur with regard to office machines in which the elements carrying the character types or the elements forming the characters are arranged, even in the rest position, very close to the printing abutment, as is true, in particular for the so-called mosaic printers, i.e. wire element or ink printers, or where the stepping movements of the typing element carrier occur at high operating speeds and are produced by relatively sensitive devices such as, for example, stepping motors or controlled d.c. motors.
For example, measures must be provided to assure that the carriage moves relatively smoothly, i.e. very uniformly, even if the relatively long guide rails are not absolutely precisely parallel and straight with respect to one another or if there is dirt, such as eraser shavings for example, on these guides.
To avoid problems of this type, it has been proposed to give pairs of axially parallel guide rollers a V-belt shape and to move them on guide rails having a circular cross section, one of the guide rollers being fastened directly and the other guide roller indirectly via a pivotal lever arm which is continuously under the influence of a spring, as disclosed in German Offenlegungsschrift No. 2,159,647. With this arrangement it is possible to avoid occasionally occurring slowdowns in the movement of the carriage guide since one of the guide rollers is yieldably supported.
But this arrangement still has the drawback that the typing element carrier is mounted without play in only one direction, i.e. in the tensioning direction of the yielding guide roller, since it is practically impossible to manufacture the guide rollers and their bearing shafts precisely enough, at commercially justifiable cost, so that they move without play in their axial direction. However, in some printers, e.g. ink printers, such bearing play can already have a damaging effect with respect to quality of the printed end product. Also, such play between parts can produce humming noises in the entire incremental drive system for the typing element carrier, which noises have a particularly annoying effect if the carriage movement has been made almost noiseless by, for example, stepping motor drives and if printing itself is also noiseless, as for example if it is effected by an ink jet method.
Other carriage guides composed of three guide rollers cooperating with one of the guide rails so as to enclose the same in the form of a star, with one of the rollers being resiliently mounted, overcome the above-noted drawback, but have the other drawback that the carriage can be mounted on or removed from the guide rails only under difficult conditions. In other words, the carriage can be mounted onto the machine frame practically only together with the guide rails.
A further known carriage guide, disclosed in German Offenlegunggschrift No. 1,817,850, has a prismatic guide rail on which two crossed pivot bearings move on guide rollers provided on the carriage. A further roller is mounted on a perpendicular shaft to follow a horizontal effective curve which cooperates with these guide rollers and with a second guide rail having a circular cross section to bring the carriage into contact with the first guide rail in that the second guide rail is mounted to be pivotal parallel to the first rail about a pivot bearing and is always under the influence of a spring. Thus the carriage can be placed onto the guide rails or removed therefrom at any point since the second guide rail can be pivoted out of an associated guide fork of the carriage. This construction however again requires very precise manufacture of the individual parts in order to prevent clatter. Between the guide fork and the guide rail there may again develop dirt accumulations which can lead to occasional heavy movement of the carriage guide.